In general, a substance is known to undergo thermal expansion in association with an increase in temperature. Thus, parts used in a device which undergoes temperature change (e.g., an electronic instrument or a precision instrument) may cause various problems due to their thermal expansion.
There have been proposed methods for reducing thermal expansions occurring at different temperatures, such as a method involving combination of a positive thermal expansion material and a negative thermal expansion material.
Patent Document 1 describes use of a negative thermal expansion material, such as a ceramic or glass ceramic material having a negative linear expansion coefficient of −1×10−6/° C. to −12×10−6/° C. within a temperature range of −40° C. to 100° C. Examples of the ceramic or glass ceramic material include a ceramic or glass ceramic material containing, as a main crystal phase, a β-quartz solid solution or a β-eucryptite solid solution, and a polycrystalline ceramic material containing, as a main crystal phase, a phosphate tungstate or tungstate containing at least one of Zr and Hf.
Patent Document 2 describes that an anti-perovskite manganese nitride represented by, for example, Mn3Zn1−xGexN (x=0.3 to 0.5) exhibits a negative linear expansion coefficient of −30×10−6/° C. within a temperature range of 51° C. to 104° C. Patent Document 2 discloses a low thermal expansion material or negative thermal expansion material containing such a nitride, and a thermal expansion reducing method involving the use of such a nitride.
However, hitherto known negative thermal expansion materials pose problems (e.g., low degree of negative thermal expansion, and narrow operation temperature range for negative thermal expansion) and have only limited applications. The practical use of such a conventional negative thermal expansion material requires various conditions, and the material is used in a narrow range of applications. Thus, such a material is not satisfactory as a thermal expansion inhibitor.
As has been known, when a ruthenium oxide represented by Ca2RuO4 and having a layered perovskite crystal structure undergoes phase transition at about 90° C., from a high-temperature metal phase (high-temperature L phase) to a low-temperature insulator phase (low-temperature S phase), the volume of the low-temperature phase is larger than that of the high-temperature phase (Non-Patent Documents 1 to 5). For example, precise structural analysis of Ca2RuO4 shows that a decrease in temperature from 127° C. to −173° C. causes a total volume variation ΔV/V (expansion) of about 1% (Non-Patent Document 3). As used herein, the term “total volume variation ΔV/V” refers to a value obtained by the formula (Vmin−Vmax)/Vmax, wherein Vmin represents the volume at Tmin (within a temperature range of negative thermal expansion from Tmin to Tmax), and Vmax represents the volume at Tmax. It has been reported that Ca2Ru0.933Cr0.067O4 (prepared through substitution of a portion of Ru of Ca2RuO4 by Cr) exhibits volume expansion (total volume variation ΔV/V=0.9%) caused by a successive decrease in temperature (Non-Patent Document 4), and Ca2Ru0.90Mn0.10O4 exhibits a negative thermal expansion of −10×10−6/° C. (ΔV/V E≈0.8%) within a temperature range of −143° C. to 127° C. (Non-Patent Document 5).
However, none of the ruthenium oxides exhibiting the aforementioned phenomena can be used as a highly functional, industrial thermal expansion inhibitor, for the following reasons: a generally narrow transition width of 1° C. or less during sharp primary phase transition, and lack of large negative thermal expansion showing a total volume variation more than 1%.